1. Field of Invention
This invention relates generally to methods and systems for measuring mechanical properties of materials, and more specifically to improvements in microindentation methods for measuring mechanical properties of materials.
2. Discussion of the Background
An Atomic Force Microscope ("AFM"), as described in U.S. Pat. No. RE 34,489, by Hansma et al., is a type of scanning probe microscope ("SPM"). AFM's are high resolution surface measuring instruments. Two general types of AFMs are relevant here, the contact mode (repulsive mode), and the intermittent contact mode AFM.
The contact mode AFM is described in detail in U.S. Pat. No. RE34,489 by Hansma et al. This AFM operates by placing a sharp tip attached to a bendable cantilever directly on a surface and then scanning the surface laterally. The bending of the lever in response to surface height variations is monitored by a detection system. Typically, the height of the fixed end of the cantilever relative to the sample is adjusted with feedback to maintain the bending at a predetermined amount during lateral scanning. The adjustment amount versus lateral position creates a map of the surface. The deflection detection system is typically an optical beam system as described by Hansma et al. Using very small microfabricated cantilevers and piezoelectric positioners as lateral and vertical scanners, AFMs can have resolution down to the molecular level, and may operate with controllable forces small enough to image biological substances.
The intermittent contact mode AFM utilizes oscillation of a cantilever to, among other things, reduce the forces exerted on a sample during scanning. This type of AFM is described in U.S. Pat. Nos. 5,226,801, and 5,415,027 by Elings et al. In U.S. Pat. No. 5,412,980 by Elings et al, an atomic force microscope is disclosed in which a probe tip on a cantilever is oscillated at or near a resonant frequency and at a predetermined amplitude called the setpoint and is scanned across the surface of a sample in intermittent contact with the sample. The setpoint is the value of the amplitude of oscillation of the cantilever which is desired while imaging the sample in intermittent contact mode. The amplitude of the cantilever is kept constant by feedback at a value equal to the setpoint. (Similarly, for contact mode, the setpoint is the value of the deflection of the cantilever which is desired while imaging the sample and is kept constant by feedback.) The amplitude of oscillation of the probe is kept constant through feedback which servos the vertical position of the cantilever mount or sample so that the probe follows the topography of the sample surface. The probe's oscillation amplitude is usually greater than 20 nm to maintain the energy in the lever arm much higher than the energy it loses in each cycle by striking the sample surface. This prevents the probe tip from sticking to the sample surface. Sample height data is obtained from the Z actuator control signal produced to maintain the established setpoint.
3. Description of the Related Art
The problem is to perform very small indentations into only the surface of a sample to measure just that surface's mechanical properties, such as hardness and elasticity. For instance, a substance such as diamond-like carbon may be deposited as a 10 nm thick film to give a hard surface to objects such as magnetic disks for data recording. It would be useful to be able to measure the hardness, wearability, adhesion, thickness and topography of these deposited films without inadvertently measuring the hardness of the substrate on which the film is deposited. Thus, accuracy in the depth of the indentation is critical to avoid penetration of the substrate by the tip or to wear and scratch only the film during a measurement.
The related art can provide accuracy by pairing a force/displacement transducer that uses columnar compression to produce force, rather than bending of a cantilever, with an STM or an AFM, as described in U.S. Pat. No. 4,848,141 by Oliver et al. Having made such a small indentation, it is both difficult to find the indentation on the surface in order to measure it, and difficult to measure if it can be found. See: Lilleodden et al., "In Situ Imaging of .mu.N Load Indents into GaAs", J. Mater. Res., Vol. 10, No. 9, 9/1995, and Bhushan et al., "Nanoindentation hardness measurements using atomic force microscopy", Appl. Phys. Lett. 64 (13), 28 Mar. 1994. The dents made by nanoindentation are usually so small that they cannot be imaged except by AFMs. Prior methods of indentation using non-AFM indenters require that the sample be moved from the indenter to an AFM to image the dent. To deal with these problems, both Lilleodden et al. and Bhushan et al. used the same tip for indentation and imaging. It is advantageous to use the same tip because the indent area remains in a known position rather than being lost while the tips are changed (Lilleodden et al.).
However, the related art encountered the further problem that using the same force detection transducer for both indentation and scanning is difficult because the optimal forces for each are orders of magnitude apart. For indentation, one may need 1000 times the force as is necessary for a non-destructive scan over the surface. According to the related art, to be able to use the same tip for both indenting and measurement, one must compromise by having either an artificially low indentation force, or a stiff cantilever that applies a high scanning force that is destructive to the sample.
Thus, it would be advantageous to have a method where the same tip could be used to apply large indentation forces to hard surfaces, and to apply very small forces for imaging the surface of the sample with high sensitivity.
As will be described, the current invention achieves high sensitivity through oscillation of the probe tip. As discussed above, Elings et al. discuss an intermittent contact, "tapping," AFM in U.S. Pat. Nos. 5,412,980 and 5,415,027 that provides sufficient sensitivity, but this has not previously been used in conjunction with indenting. Nano Instruments provides an indenter that oscillates its indenting tip while indenting. As used by Oliver et al., however, the tip is not used to image the sample, is not mounted on a cantilever, and is not necessarily oscillated at its resonant frequency. Oliver et al., "An Improved Technique for Determining Hardness and Elastic Modulus using Load and Displacement Sensing Indentation Experiments," J. Mater. Res., Vol. 7, No. 6, June 1992. Thus, the oscillation of the indenter is not used to reduce the force exerted on the sample during imaging.
There is also significant interest in determining the toughness, film adhesion, wearability and durability of thin films on a very small scale. Scratching and wear testing can yield data on film adhesion values, toughness/durability of films, and thickness of the films. However, for the same reasons that it is difficult to use the same stiff cantilever to make a dent and to then image the dent, it is also difficult to use the same tip to scratch or wear hard samples and then apply a small force to image the sample just scratched or worn. The scratches, like dents, are usually so small that they cannot be imaged except with an AFM. Thus, the advantages of indenting discussed herein apply equally to both scratching and wear testing. It is preferable to make an image of the scratch or wear test to examine the nature of the scratch or wear. For instance, when testing a thin film, it may be advantageous to see if the film lifts off the surface, i.e., to check its adhesion or to measure the thickness of the lifted edge.